UK light-heavyweight Buatsi secured a victory yet disappointing debut to life under recent management with a points victory over Parker in the Manchester arena.
The Olympic medal winner's work rate likely gained him a majority decision in a tight ten-round bout at the boxing arena.
But Parker landed significant shots, connecting with powerful clean blows in a fight where the approaches never quite clicked, with excessive holding disrupting the contest.
The scoring judges scored it 96-94 for to Buatsi, with the final judge a 95-95.
"It wasn't my finest performance but what defines champions? They succeed," the victor stated in the ring as he was greeted with boos from the spectators.
The Londoner returned to winning ways after his maiden career setback - an epic battle with his previous opponent in February - to secure a 20th career victory.
Several boxing experts thought Parker, thirty-one, deserved the decision but the contender endured the second setback of his 28-match career.
The Champion Secures Win in Uneventful Affair
His career development has slowed in recent times through contract problems and long spells of lack of fights.
Once tipped as the future of the British fight scene, he now is positioned trying to achieve expectations - possibly in what feels like a final roll of the dice - under fresh management.
He began cautiously, with both competitors willing to be aggressive in a cagey early stages.
The challenger, whose sole prior loss came at super middleweight against John Ryder in 2022 after breaking his hand during the bout, was participating in his second bout at the higher weight class.
By the fight's midpoint, Parker had kept it close, causing problems for the favorite, who utilized a protective stance to edge the better stanzas.
The audience's tolerance decreased amid regular grabbing, with the challenger falling to the floor several times during close exchanges.
The underdog ended stronger, but several stanzas were challenging to assess. However, former world champion Carl Frampton - working with Dazn - had it significantly in favor for the challenger, giving Buatsi just only one stanza.
Buatsi advances, but with the likes of heavy hitter David Benavidez, and Russian champion Dmitry Bivol ruling the division, the road to world glory - based on this performance - still seems difficult.
Hatton Memorial Inspires Local Rivalry Contest
In the supporting bout, Arthur overcame Bradley Rea by divided decision in a excellent all-Manchester battle to secure the continental title.
The arena became quiet for a deeply poignant homage before the fight as 10 bells tolled in honor of Manchester's own Ricky Hatton, who succumbed aged 46 in last month.
It was the initial significant broadcast match card in the urban center since Hatton's passing. As the last bell faded, the quiet became a chant of "there's only one Ricky Hatton".
The competition inside the ropes reflected the feeling. The eventual winner was knocked down by a significant blow flush on the face in the early round, rising shakily to his feet on the referee's count.
But the red-colored equipment of the fighter soon advanced again. Across the ring, Rea - in opposing blue - was injured soon, significant bruising forming under his eye as the fighters exchanged powerful shots.
Rea, 27, landed power punches but Arthur, thirty-four, fought fire with fire with powerful upward punches as the intensity seldom decreased over twelve rounds.
Arthur narrowly won the decision with official scores of 114-114, close margin and wider gap, but each competitor were persistent and full of heart, all the qualities their hero Hatton stood for.
